Multi-range hydro-mechanical transmissions are known for providing smooth shifting between various different gear ratios in machines and other vehicles. Shifting in a typical hydro-mechanical transmission is accomplished by the use of two or more clutches that can synchronously disengage one gear ratio while simultaneously engaging another gear ratio. The synchronization of clutch speed can be achieved by use of synchronizing assemblies, which can be embodied in various forms. One known structure for a synchronizing assembly includes gears connected to clutches that are configured to selectively engage two or more rotating elements in a transmission such that a rotating element associated with the gear to be engaged is made to rotate at the same speed as a rotating element associated with the gear already engaged. In this way, the transmission of torque and power between gears can be shifted smoothly and without an abrupt step.
One example of a known hydro-mechanical transmission can be seen in U.S. Pat. No. 7,530,913 (the '913 patent), which was granted on May 12, 2009. The '913 patent describes a “Multi-Range Hydromechanical Transmission” that includes an input member, a hydrostatic transmission and a mechanical transmission. The mechanical transmission of the '913 patent includes first and second synchronizing assemblies for synchronizing first or second output members of the transmission to a combined output speed from the input member and the hydrostatic transmission. In one embodiment of the transmission described in the '913 patent, first and second clutch assemblies alternately engage to transfer power from the synchronized output member to a final drive.
Although the transmission described in the '913 patent effectively shifts between forward and reverse gears, it includes three synchronizers and two clutches when embodied in a transmission having five gear ratios, for example, three forward and two reverse gears. During operation, one synchronizer and one clutch are engaged for each gear ratio, which means that the second clutch and the remaining two synchronizers are idle. Thus, each gear change requires the engagement of one of the idle synchronizers and the disengagement of another. Such synchronizer engagement and disengagement at each gear shift can cause wear in the friction material of the synchronizers, especially for high-torque applications such as in heavy trucks and earth working machines and other heavy machines. Moreover, the relative complexity of synchronizers decreases the reliability and increases the cost of the transmission.